Please don't restrict the editing of posts. Post editing can be invaluable for correcting stupid mistaks you make. Like you post something you thought was well thought-out (and you read it over before you post it), but when it's posted you find it was too aggressive and you can go back and edit it so it doesn't provoke a fight.

Also, I'm against locking the first post of a thread. I started a thread which got me ridiculed and it became obvious to me that one reason I was getting so much ridicule was that people were misunderstanding the view I was presenting. So I went back to the first post. I didn't do what you are clearly worried about. I didn't edit out anything from it. I didn't remove any text. I just added an end note explaining my viewpoint better. The reason posting it later in the thread wasn't good enough is that people were clearly coming to the thread and reading the first post and then posting a reaction rather than reading all my explanation way later on in the thread. I wanted an explanation that would be read along with my first post, to counter the negative impression the first post gave. I didn't want a post sitting there encouraging people to laugh at me for all eternity. I've had some very uncomfortable experiences on this site and the ability to edit that post made me feel a bit better. My approach does not remove anything but allows correction. I clearly labeled my edited-in end note as something that was added in later, complete with a date.

Now if you made it impossible to take text out of a first post but not impossible to add text then I could still do what I did. So please if you do lock first posts, only do so partially and allow addition of new text at the end of the post.